One method for forming a continuous sheet of a rigid thermoplastic material involves continuously extruding a heated thermoplastic material which is fed between a pair of pull rolls each of which rotates about a generally horizontal axis so that there is a lower roll and an upper roll. After passing between the pair of pull rolls, the extruded thermoplastic material moves with the upper roll and passes between the upper roll and a texturizing roll which also rotates around a generally horizontal axis and is located above the upper pull roll. The upper roll and the texturizing roll ensure that the extruded thermoplastic material moves with the outer peripheral surface of the upper roll until it moves between the upper roll and the texturizing roll. All of the rolls are temperature controlled by flowing a cooling fluid, such as water having solubles contained therein, through the rolls to control the temperature of the outer peripheral surfaces thereof. After passing between the upper roll and the texturizing roll, the extruded thermoplastic material moves with the outer peripheral surface of the texturizing roll and is removed therefrom and pulled over a cooling conveyor by another pair of pull rolls. When it was desired to use the above-described apparatus for forming a continuous sheet of a rigid thermoplastic material having a plurality of recesses formed in one surface thereof and a plurality of projections on the other surface thereof, there existed the problem of how to form the recesses and projections and to maintain them in a desired shape while cooling the thermoplastic material to a temperature at which the thermoplastic material could be pulled over the cooling conveyor without damaging the recesses and projections formed therein.